Magnetic disk storage units, commonly referred to as hard drives, are the principal mass storage systems utilized in personal computers, servers and larger computer systems for the non-volatile storage of programs and data files. The speed, capacity and quality of modern hard drives has never been higher, yet the physical size and cost of hard drives has never been smaller. A typical hard drive today generally has a service life of five years. The construction of a hard drive, and organization of data therein, are illustrated in FIGS. 1 and 2, a discussion of which follows.
Data is organized on the surface of a magnetic disk as shown in FIG. 1. The disk drive unit includes a circular disk, referred to as a platter, 102 having its surface coated with a magnetizable material and a read/write head 104 attached to a movable arm 106. Data is recorded onto the surface of the platter in a concentric set of rings T0 through T3, called tracks. Arm 106 is movable in the directions indicated by arrows 108 to position head 104 over any one of tracks T0 through T3. Each track is seen to be divided into sections identified as sectors, wherein blocks of data are stored, each block containing 512 bytes of data. The sectors corresponding to tracks T0, T1, T2 and T3 have been numbered S0 through S9, S10 through S19, S20 through S29, and S30 through S39, respectively. A second read/write head, not shown, may be provided to provide access to the bottom surface of platter 102.
A disk stack, shown in FIG. 2, consists of multiple platter 202 through 208 affixed to a common shaft or spindle 220. Each platter is similar in construction to disk platter of FIG. 1. Multiple read/write heads H1 through H7 provide access to platter surfaces 202A, 202B, 204A, 204B, 206A, 206B, 208A and 208B, respectively. The heads are moved in unison in the directions indicated by arrows 210 to locate corresponding tracks on each platter. The corresponding concentric tracks on platters 202 through 208 are referred to as cylinders.
The platter or disk stack is rotated at constant speed during operation, typically at 7200 revolutions per minute, with each read head/write head riding over the surface of its corresponding platter on a cushion of air 1 to 2 millionths of an inch deep. To read or write information, the read/write head must be positioned contiguous to the desired track and at the beginning of the sector to be accessed. Access time includes the time it takes to position the head at the desired track or cylinder, known as seek time, and the time it takes for the head to line up with the sector to be accessed, known as rotational latency. Seek time can be eliminated by providing a fixed head for each track, but modern drives have thousands of tracks which makes this technique impractical.
During manufacture of a hard disk drive, a small number of sectors will invariably contain defects. These permanent defects are discovered during testing by the drive manufacturer and identified in a Primary Defect List (P-List) provided with the hard disk drive. All modern disk drives also include the ability to identify sectors which become damaged or doubtful during the use of the drive, and to remap the bad sectors, automatically locating a new sector to store the data otherwise stored on the bad sectors. The location of sectors identified as damaged during use following the manufacture of the disk drive are maintained in a second list referred to as a Grown Defect List (G-List).
SCSI (Small Computer Systems Interface) Hard Disks include a Read Defect Data command in which the user may read the Grown Defect List. A Grown Defect is the address of a block that has been reassigned, either automatically due to a hard media error or by the Reassign Blocks command. A G-List entry may be read in different formats, but all current NCR Corporation qualified disk drives support, at least, the Physical Sector Format: Cylinder, Head, and Sector, as defined in the SCSI-2 specification. Each reassigned block is one entry in the G-List. G-List entries are not time stamped. Several utilities are available that may be used to read the G-List. For example, FIG. 3 provides a windowed display of the primary and grown defect lists for a SCSI hard drive generated by Adaptec, Incorporated's. SCSI Interrogator program.
The total number of reassigned blocks that a disk drive can support is determined by the particular scheme that the drive manufacturer uses for the disk drive model. However, the NCR Generic Requirements Specification for 31/2" SCSI Disk Drives requires that no more than 0.01% of the total number of blocks on a drive be reassigned during the service life of a disk drive. The service life is, generally, five years.
A method for identifying and replacing hard disk drives prior to drive failure is desired. It is believed that in many cases, a potential hard disk drive failure can be identified by monitoring the growth of the Grown Defect List for the disk drive.